The present invention relates to a novel roadway luminaire which is useful in that it may be adjusted to conform to a particular standard of illumination.
Luminaires which are employed in earlier lighting system are generally of the "box cut-off" and "prismatic lens" types. The "prismatic lens" type luminaire relies on refraction of light rays from the source lamp to direct light to the high angle zone generally defined between 65 and 75 degrees in relation to a line projected downwardly from the luminaire which is perpendicular to the ground surface. The "prismatic lens" system has a tendency to produce light at higher than normal viewing angles (generally above 75 degrees) which are undesirable and often termed as "glare" within the normal angles of vision of a motorist.
The "box cut-off" luminaire such as the type shown in the U.S. Pat. No. 4,053,766 greatly reduces glare at normal viewing angels while still producing a broad distribution of light similar to that of "prismatic lens" type luminaire. However, the "box cut-off" luminaire suffers restrictions in that the portion of the reflector directing light to the high angles of emission is quite small, about a 30 degree sector around the light source. Thus, the remaining reflectors are restricted to directing light to the lower angles of emission which may produce an undesirable result, by creating disuniform levels of illumination on the ground surface ("hot spots").
Moreover, the standard for roadway and area lighting may be described by the "illuminance" system or the "luminance" system. The former systems depends on the amounts of light falling on a ground surface and the latter depends on the he light reflected by the ground toward the viewer. As may be apparent, the "luminance" standard thus takes into consideration the different levels of specularity of the road surface. For example, a diffuse surface tends to reflect light in all directions and requires a pattern of distribution from a lighting system which is very much like the one for the "illuminance" system. On the other hand, a highly specular roadway surface, one which tends to reflect light outwardly from the source must be reinforced on the downstream side of the luminaire and diminished on the upstream side of the luminaire to produce a more uniform luminance on the surface, as viewed by a motorist driving in that direction.
A roadway luminaire which addresses and solves these problems encountered in the lighting field would be a great advance therein.